The present invention relates to a method of tightly bonding two substrates.
Conventionally, a joint substrate which is formed by tightly bonding two substrates is used widely. For example, there is known an SOI (silicon on insulator) substrate which is formed by joining two semiconductor substrates with an oxide film (SiO2) interposed therebetween and thinning the semiconductor substrate on the activation side. The SOI substrate is used as a substrate for a semiconductor device having effects of a high withstand voltage, a high speed, etc.
When two substrates are overlaid, the substrate on the top is in a state floating on a gas present on the lower substrate. Therefore, friction produced when the top substrate moves on a horizontal plane is very small, so that the two substrates are displaced even if there is a little inclination. To align the two substrates accurately without displacing them, it is necessary to provide means for limiting the movement of the upper substrate.
As means for controlling the movement of the substrates, for example, a plurality of positioning pins 201, 202, 203 and a pressing pin 204 are disposed around the two overlaid substrates 101, 102 positioned on a jig 200 to control the horizontal movement of the substrates as shown in FIG. 6. They are bonded by being left standing for a predetermined time.
But, when the movement of the substrates 101, 102 is controlled by the plurality of positioning pins 201, 202, 203 and the pushing pin 204 disposed around them, the substrates are in contact with the pins, and joining of the substrates are disturbed at the contact portions, possibly resulting in a disadvantage such as occurrence of voids or particles.
For example, when the movement of the substrates 101, 102 is controlled by pushing them by the pushing pin 204 and its pushing force is excessive, joining of the substrates 101, 102 is impaired around the contact portions with the pushing pin 204, causing a problem that voids are produced.
When the number of pins for positioning is decreased, an orientation flat section (hereinafter called xe2x80x9cOF sectionxe2x80x9d) of the two substrates is displaced or a notch section is displaced, causing a disadvantage that a rate of defective products increases.
When joining of the two substrates is started from many joining points on a joining interface between the two substrates, there is a problem that voids tend to be produced on the joining interface. This method also has a drawback that its bonding time is long because the substrates are left standing to join together.
Therefore, it is an object of the present invention to provide a method for bonding substrates which can make accurate positioning, does not produce voids or the like and has a short bonding time.
The present invention is a method for bonding substrates to tightly bond two overlaid substrates which comprises the steps of:
aligning the substrates, freely falling a second substrate onto a first substrate to overlay the first and second substrates;
interposing an air layer between joining surfaces of the first and second substrates, aligning the first and second substrates and holding the aligned first and second substrates; and
pressing one point of the aligned two substrates.
Thus, interposing the air layer between the joining surfaces of the first substrate and the second substrate facilitates the movement of the second substrate to be placed on the first substrate and enables free movement of the second substrate. Thus, the two substrates can be aligned accurately.
By pressing one point of the two substrates which are accurately aligned, a high quality product can be produced without producing voids or the like on the joining interface between the two substrates and a bonding time can be shortened unlike the case that joining is started from a plurality of joining points.
The present invention is a method for bonding substrates, wherein in the above step of interposing the air layer between the joining surfaces of the first and second substrates, aligning them and holding the aligned first and second substrates, the alignment is made by pressing at least one of the first and second substrates to form the air layer between the joining surfaces of the first and second substrates.
For example, when the second substrate is freely fallen to be placed on the first substrate, the joining surfaces of the first substrate and the second substrate are occasionally adhered at several points. According to the present invention, when at least one of the first and second substrates is pressed to move in order to align them, the several joined points adhering the first substrate and the second substrate are separated to form a constant air layer.
The present invention is a method for bonding substrates, wherein in the step of interposing the air layer between the joining surfaces of the first and second substrates, aligning them and holding the aligned first and second substrates, the first substrate is placed on a mounting table by adsorbing; and the adsorption of the first substrate is released to form the air layer between the joining surfaces of the first and second substrates.
Thus, when the first substrate is adsorbed to position on the mounting table, it is caused to swing by releasing its adsorption, the second substrate which is partially adhered to the first substrate is separated, and a constant air layer is formed between the first and second substrates.
The present invention is a method for bonding substrates, wherein in the step of interposing the air layer between the joining surfaces of the first and second substrates, aligning them and holding the aligned substrates, the alignment is performed by applying a pressing force to at least a part of the substrates, and the substrates are held. Then, the pressing force is lower than a force of naturally adhering the first and second substrates.
When the pressing force applied to the first and second substrates to align and hold them is higher than the force of naturally adhering the two substrates, joining at the portion to which the pressing force is applied is impaired, resulting in causing voids. Therefore, a high quality bonded substrate free from voids can be obtained by aligning and holding the substrates by the pressing force which does not impair the joining of the first and second substrates, namely a pressing force not producing voids.
According to the present invention, the air layer interposed between the joining surfaces of the first and second substrates is uniform.
Thus, when the air layer interposed between the joining surfaces of the first and second substrates is uniform, a high quality bonded substrate free from voids can be obtained.